The paper presents the first data on the crystal chemistry of the cation-exchanged forms of layered titanosilicates belonging to the epistolite group. It was found that these heterophyllosilicates have high exchange capacity and selectivity for cations of chalcophile elements (Ag, Cu and Zn) and could be considered as potentially novel raw materials or, more likely, as possible prototypes of cation-selective synthetic microporous materials. The crystal structures of Ag- and Cu-exchanged forms of lomonosovite and Ag-exchanged form of murmanite were studied by single-crystal X-ray diffraction. The topology of the main structural unit, the HOH block, remains unchanged in cation-exchanged forms as compared to the initial lomonosovite Na4Ti4(Si2O7)(2)O-4 center dot 2Na(3)PO(4) (P-1) and murmanite Na4Ti4(Si2O7)(2)O-4 center dot 4H(2)O (P-1). In Ag-exchanged murmanite, Ag cations occupy two crystalographically non-equivalent positions: one in the heteropolyhedral (H) sheet and another one in the octahedral (O) sheet